Title: Tumbled OpalsImage: Created by Aaron J. Greenblatt using an Epson Perfection 1200U scanner.Editing: Edited in PhotoShop 7.0 for color accuracy, size, and to apply copyright and border.

Location: These opals currently reside in my private collection in the lovely state of Michigan, USA.

Description: These are natural black, brown, and crystal opals from Australia. They have been tumbled but not polished. Most of these opals are smaller than a US quarter. They have not been chemically treated or enhanced in any way, aside from being tumbled.

These opals display a wide range of colors and patterns and most of the time the colors go all the way through the stones. Under just normal lighting conditions, this pile of opals radiates with dull splashes of color in all directions. This image doesn't do these stones any justice. Because these pieces aren't polished, they still look like pebbles one might find on a beach. The difference of course is their bright changing patterns and colors. Try to imagine an entire beach full of nothing but these types of stones. It would be incredible to behold.

About Precious Opal: Precious opal shows a play of colors in white light that is due to diffraction from the regular packing of clathrate-like silica gel spheres. Clathrate is a chemistry term referring to a substance in which a molecule of one compound fills a cavity within the crystal lattice of another compound. In the case of opal, it's water spheres filling a crystal lattice of silica.

Although the packing of the spheres may be regular, there is neither short-range nor long-range order in this material, and the X-ray powder diffraction pattern is characterized by a distinct broad hump and a possible weak second hump indicative of material that is "X-ray amorphous". What that means is that opal has no large crystalline structure.

According to Mindat.org, precious opal should properly be labeled as "Opal-AG". The subscript G has been added to indicate that the structure of the silica network is gel-like in that it is composed of large cages with included water essential to the stability of the structure. Most but not all precious opal is opal-AG. So basically, if you see an opal that has flashes of color, that color is caused by spheres of water within the amorphous silica structure and thus should be labeled Opal-AG rather then being called precious opal. Of course, none of this is going to stop anybody from calling opal that has colors "precious opal" but it's still interesting information to know.

What about common opal? Opal that shows no play of colors is also composed of the same spherical clusters, but they are non-uniform in size and do not pack in an orderly manner thus destroying any possible diffraction of the light and thus any cool coloration. This type of opal is termed collectively "potch opal" and includes several massive varieties.

These opals look like the sort that come from Coober Pedy, South Australia - the state I used to live. Even there though, tumbled opals were not very common - stones that colourful were most commonly cut into low-grade cabs or even doublets/triplets for tourists. Kudos on this find!

Coober Pedy and Andamooka are the two biggest locales for white opal - the stones from both look very similar. That's about all I can say. Although I think the Coober Pedy source is more likely to also produce opalised Jurassic fossils.

You most certainly could XD If you don't like dealing with the critters, there are tourist places where you can pick through the stuff that comes out of conveyor belts, and you *WILL* get a lot of stuff that the workers miss.

Even precious opal (at least in terms of raw, but glittery veins in matrix) is reasonably common such that at one point my old gem club just gave them as 1 kilogram bags to children for free, and we have several heavy bucketsful of unsorted stuff from which I regularly pulled enough material for small shapes and cabs.

Hmm, I'm not so sure that if I was a mine owner that I'd keep around workers who miss too much good stuff.

I understand what you mean about the material being cheaper at its source. I think the same is true just about anywhere. Like here in Michigan we can find baseball-sized Petoskey stones (a type of fossilized coral gemstone found only in MI) just washing up on the public beaches for free. If I take those same stones and drive them down to Texas, I can sell them for $20 a stone or more.

"So basically, if you see an opal that has flashes of color, that color is caused by spheres of water within the amorphous silica structure"

I've read that the colors in opal are due to the size of the silica spheres in the opal and their arrangement. This would make more sense since Gilson and other "synthetic" opals contain no water but still have play of color. These so called "synthetics" should actually be labeled as imitation or simulated since they don't contain water and are usually bound by an epoxy resin. This would also explain why some opals only show some colors. I believe the small spheres cause blue and large spheres cause red the rarest and most expensive opal. Thought this might interest you